The answer is II and IV. Von Thunen's Theory 's proncipal variable is distance. If the crop field is close to the consumption area (a city), the cost of that land will be expensive. If it is far away, the cost of the land will reduce but the transportaions ones will increase. With the improvement of transportation the distance won't be such a determinant factor, those costs reduce considerably.
With the improvement of crop yields, wich Von THunen did not had in consideration, a single crop can provide a lot of food making the cost of the land reduce its importance.
